1. Field of the Invention
The present invention relates generally to a vehicle control system and method using optical tracking, and in particular to such a system and method for an agricultural vehicle including a towed component comprising an implement.
2. Description of the Related Art
Automatic control of steering (“autosteering”) of vehicles is becoming more widespread, especially in agricultural and mining applications. Most commercially available automatic steering systems include a controller that has means for determining, among other things, the position and heading of a vehicle, a computer-based system for comparing the position and heading of the vehicle with a desired position and heading, and a steering control responsive to a control signal issued by the controller when the position and/or heading of the vehicle deviates from the desired position and/or heading.
As used herein, “attitude” generally refers to the heading or orientation (pitch with respect to the Y axis, roll with respect to the X axis and yaw with respect to the Z axis) of the vehicle, or of an implement associated with the vehicle. Other vehicle/implement-related parameters of interest include groundspeed or velocity and position. Position can be defined absolutely in relation to a geo-reference system, or relatively in relation to a fixed position at a known location, such as a base station. A change in one or both of the position and orientation of the vehicle (which can include a towed component, such as an implement or a trailer) can be considered a change in the vehicle's “pose.” This includes changes (e.g. different order time derivatives) in attitude and/or position. Attitude and position are generally measured relatively with respect to a particular reference flame that is fixed relative to the area that the vehicle is operating in, or globally with respect to a geo-reference system.
Automatic control systems for controlling steering of a vehicle may include a global navigation satellite system (GNSS, including the global positioning system (GPS)) based system. GNSS-based systems typically include a GNSS receiver mounted on the vehicle that receives signals from constellations of GNSS satellites that orbit the earth. The GNSS receiver can then determine or estimate a location of the vehicle. A number of early automatic steering control systems included GNSS-only systems. These systems suffered from limitations in that signals from the constellation of GNSS satellites are received at a relatively low rate, meaning that the location of the vehicle was also determined from the GNSS system at a relatively low rate. As the vehicle is continually moving, there were significant periods during which the location of the vehicle was not being determined. Accordingly, the vehicles would often deviate from the desired path of travel.
Significant work has also been conducted in respect of using inertial sensors to attempt to control the steering of the vehicle. Inertial sensors include accelerometers and/or gyroscopes that can be used to provide indications as to the attitude and speed (or changes thereto) of the vehicle. Unfortunately, inertial sensors such as accelerometers and gyroscopes suffer from time-varying errors. This is particularly marked in the less expensive inertial sensors used in commercially available vehicle steering control systems. Less expensive inertial sensors are used in commercially available systems principally to reduce the cost of the systems to make them affordable.
U.S. Pat. No. 6,876,920, which is assigned to a common assignee herewith and incorporated herein by reference, describes a vehicle guidance apparatus for guiding a vehicle over a paddock or field along a number of paths, the paths being offset from each other by a predetermined distance. The vehicle guidance apparatus includes a GNSS receiver for periodically receiving data regarding the vehicle's location, and an inertial relative location determining means for generating relative location data along a current path during time periods between receipt of vehicle position data from the GNSS receiver. The apparatus also includes data entry means to enable the entry by an operator of an initial path and a desired offset distance between the paths. Processing means are arranged to generate a continuous guidance signal indicative of errors in the attitude and position of the vehicle relative to one of the paths, the attitude and position being determined by combining corrected GNSS vehicle location data with the relative location data from the inertial relative location determining means.
In the system described in U.S. Pat. No. 6,876,920, the inertial sensor is used to provide a higher data rate than that obtainable from GNSS alone. Although the inertial navigation system (INS) part of the steering control system suffers from errors, in particular a yaw bias, the signals received from the GNSS system are used to correct these errors. Thus, the combination of a GNSS based system and a relatively inexpensive INS navigation system allow for quite accurate control of the position of the vehicle. Although this system allows for accurate vehicle positioning and sound control of the vehicle's steering, difficulties may be experienced if there are prolonged periods of GNSS outage. GNSS outages may occur due to unsuitable weather conditions, the vehicle operating in an area where GNSS signals cannot be accessed, or due to problems with the GNSS receiver. If a period of prolonged GNSS outage occurs, the steering system relies solely upon the INS. Unfortunately, a yaw bias in a relatively inexpensive inertial sensor used in the commercial embodiment of that steering control system can result in errors being introduced into the steering of the vehicle.
Optical computer mice are widely used to control the position of a cursor on a computer screen. Optical computer mice incorporate an optoelectronic sensor that takes successive pictures of the surface on which the mouse operates. Most optical computer mice use a light source to illuminate the surface that is being tracked (i.e. the surface over which the mouse is moving). Changes between one frame and the next are processed using the image processing ability of the chip that is embedded in the mouse. A digital correlation algorithm is used so that the movement of the mouse is translated into corresponding movement of the mouse cursor on the computer screen.
The optical movement sensors used in optical computer mice have high processing capabilities. A number of commercially available optical computer mice include optical mouse sensors that can process successive images of the surface over which the mouse is moving at speeds in excess of 1500 frames per second. The mouse has a small light emitting source that bounces light off the surface and onto a complimentary metal oxide semiconductor (CMOS) sensor. The CMOS sensor sends each image to a digital signal processor (DSP) for analysis. The DSP is able to detect patterns in images and see how those patterns have moved since the previous image. Based on the change in patterns over a sequence of images, the digital signal processor determines how far the mouse has moved in X and Y directions, and sends these corresponding distances to the computer. The computer moves the cursor on the screen based upon the coordinates received from the mouse. This happens hundreds to thousands of times each second, making the cursor appear to move very smoothly.
The chips incorporated into optical computer mice often include photodetectors and an embedded integrated circuit that is used to analyse the digital signals received from the photodetectors. The photodetectors may include an array of photosensors, such as an array of charge coupled devices (CCDs).
U.S. Pat. No. 5,786,804 (incorporated herein by reference), which is assigned to Hewlett-Packard Company, describes a method and system for tracking attitude of a device. The system includes fixing a two-dimensional (2D) array of photosensors to the device and using the array to form a reference frame and a sample frame of images. The fields of view of the sample and reference frames largely overlap, so that there are common image features from frame to flame. Several frames are correlated with the reference frame to detect differences in location of the common features. Based upon detection of correlations of features, an attitudinal signal indicative of pitch, yaw and/or roll is generated. The attitudinal signal is used to manipulate a screen cursor of a display system, such as a remote interactive video system.